An image-forming apparatus known in the art comprises a heating member for heating a recording sheet, a non-contact temperature sensor separated from the heating member for detecting the ambient temperature around the heating member, and a control device for controlling the temperature of the heating member based on the temperature detected by the temperature sensor. More specifically, this conventional image-forming apparatus employs a technique to adjust the temperature detected by the temperature sensor using a prescribed function for approximating the actual temperature of the heating member and regulates the temperature of the heating member based on this adjusted temperature. In this technique, the conventional image-forming apparatus also changes the function for estimating temperature based on the detected temperature. Conventionally, one function has been selected for the range of temperatures detected during the entire print control process, from start to finish, such as the temperature range 175-200° C.
However, the conventional image-forming apparatus described above employs a non-contact temperature sensor for measuring ambient temperature around the heating member, but the ambient temperature does not closely follow the rapid rise in the temperature of the heating member that occurs during the warm-up period. Consequently, the difference between the actual temperature of the heating member and the ambient temperature (detected temperature) can be much greater at the start of print control, following completion of the warm-up operation, than during the latter half of print control. In such cases, the conventional image-forming apparatus using a single function for correcting the detected temperature during the print control process cannot control fixing operations with accuracy. That is, the conventional image-forming apparatus cannot approximate the actual temperature using a temperature corrected with the same function when the difference between the actual temperature and the detected temperature of the heating member is great at the start of print control that directly follows a warm-up operation. As a consequence, the control device may increase the amount of power supplied to the heating member, causing the temperature of the heating member to rise too high.